1. Field of the Invention
The present invention relates to a laser scanning unit for use in an image forming apparatus such as a laser beam printer or a digital copying machine. More particularly, the present invention is related to a multi-beam laser scanning unit and a multi-beam light source unit having an improved structure, capable of recording a plurality of lines simultaneously using a plurality of laser beams.
2. Description of the Related Art
Recently developed electrophotographic image forming devices are employing a multi-beam laser scanning unit for simultaneously recording a plurality of lines using a plurality of laser beams. One example of such a device includes a laser-beam printer. The multi-beam laser scanning unit simultaneously scans a plurality of laser beams spaced apart from each other. As shown in FIG. 1, the multi-beam laser scanning unit comprises a multi-beam light source unit 10, a cylindrical lens 20, a polygon mirror 30, an image resulting lens 40, a detecting mirror 51 and an optical sensor 52 for detecting a synchronous signal, and a frame 60 for receiving and supporting the above elements.
The multi-beam light source unit 10 comprises a laser diode 11 for discharging at least two laser beams P1 and P2, a diode holder 12 for fixing the laser diode 11, an operation circuit board 13 for controlling operation of the laser diode 11, a collimating lens 14 for converting a plurality of the laser beams discharged from the laser diode 11, into a parallel light, and a lens holder 15 joined to the diode holder 12, for supporting the collimating lens 14.
The two laser beams P1 and P2 discharged from the laser diode 11 are made parallel by the collimating lens 14, and illuminate a reflecting surface of the polygon mirror 30 through the cylindrical lens 20. The image reflected from the surface of the polygon mirror 30 passes through an image resulting lens 40 onto a photosensitive material of the rotational drum (not shown).
The cylindrical lens 20 linearly condenses the laser beams P1 and P2 on the reflection surface of the polygon mirror 30, whereby a point image results on the photosensitive material of the rotational drum. The point image is not distorted due to a surface slope of the polygon mirror 30. The image resulting lens 40 comprises a spherical lens and a toric lens. The image resulting lens 40 performs the functions of preventing point image distortion on the photosensitive material, which is similar to the function of the cylindrical lens 20, and amends the image so that the point image can be scanned in a primary scanning direction at a constant velocity on the photosensitive material.
The two laser beams P1 and P2 are separated, respectively, at the end of the primary scanning surface by the detecting mirror 51, and introduced to the optical sensor 52 positioned on the opposite side of the primary scanning surface. The two laser beams P1 and P2 are then converted into a recording initiation signal by the controller (not shown), and transmitted to the laser diode 11. The laser diode 11 starts recording modulation of the two laser beams P1 and P2 by receiving the recording initiation signal. By adjusting the recording modulation timing of the two laser beams P1 and P2, the recording initiation position of an electrostatic latent image formed on the photosensitive material of the rotational drum is controlled.
The cylindrical lens 20, the polygon mirror 30, and the image resulting lens 40 are mounted on the bottom wall of the frame 60. After the optical parts are mounted on the frame 60, the opening on the upper part of the frame 60 is sealed by a cover (not shown).
The multi-beam light source unit 10 is mounted on the sidewall 60a of the frame 60 as shown in FIG. 2. When the multi-beam light source unit 10 is mounted on the frame 60, the diode holder 12 is inserted into an opening 60b formed on the sidewall 60a. Following insertion of the multi-beam light source unit 10, the focus and optical axis of the collimating lens 14 are adjusted, and the lens holder 15 is fixed to the diode holder 12.
After the horizontal and vertical positions between the laser beams P1 and P2 are adjusted by rotating the entire multi-beam light source unit 10, by as much as up to a predetermined angle (θ) with respect to the optical axis, the diode holder 12 is fixed to the sidewall 60a of the frame 60 by means of a screw 61.
According to the general multi-beam laser scanning unit described above, however, the horizontal and vertical positions between the laser beams are adjusted by rotating the multi-beam light source unit 10 a predetermined angle with the multi-beam light source unit 10 only temporarily assembled to the frame 60 of the laser scanning unit. Additionally, another manufacturing process step is added because the multi-beam light source unit 10 is fixed while rotated in the frame 60 of the laser scanning unit in the assembly line of the laser scanning unit. Thus, job efficiency and productivity are negatively affected. For example, manufacturing equipment becomes large and complicated because the equipment for adjusting the horizontal and vertical positions between the laser beams from the multi-beam light source unit 10, which is a large-scale rotation adjuster having a suitable size to accommodate the laser scanning unit, must be added to the main assembly line. Lastly, adjustment is difficult to perform and therefore job efficiency deteriorates.